1. Field of the Invention
The present invention relates to a method of forming a self-aligned silicide, abbreviated as salicide, on a semiconductor wafer, and more particularly, to a method of forming a cobalt-containing salicide on a semiconductor wafer.
2. Description of the Prior Art
Self-aligned silicide, (salicide) is applied widely in the formation of a MOS transistor to reduce the sheet resistance, and hence provide the devices of the MOS transistor and the metal good ohmic contacts. In the prior art method of forming the salicide, the rapid thermal process is applied in the thermal silicidation process. However, rapid thermal technology is still developing at present time, limiting the yield of the salicide process. Therefor, how to improve the rapid thermal process to enhance the yield of the salicide process becomes a very important issue in the formation of the MOS transistor.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a cobalt-containing salicide 20 on a MOS transistor 10 according to the prior art. FIG. 2 illustrates the relation between time and temperature for the first rapid thermal process in the prior art method of forming a salicide. The MOS transistor 10 is positioned on the surface of a semiconductor wafer 22 and isolated from other devices by shallow trenches 24 formed by using the shallow trench isolation method. The MOS transistor 10 comprises a poly-silicon gate 12, a gate insulation layer 13, two spacers 14, a source 16 and a drain 18. Cobalt-containing salicides 20 are formed on the surface of the poly-silicon gate 12, the source 16 and the drain 18.
In the prior art method of forming the cobalt-containing salicide 20, a cobalt-containing metallic layer 26 is deposited on the MOS transistor 10, and then a first rapid thermal process is performed to rapidly heat the semiconductor wafer 22 up to 400.about.680.degree. C. The high temperature is maintained for 10.about.40 seconds and then cooled down. This first rapid thermal process allows a portion of metals in the cobalt-containing metallic layer 26 react with poly-silicon of the gate 12 and silicon of the source 16 and the drain 18, resulting in the formation of CoSi or Co.sub.2 Si.
As shown in FIG. 2, during the first rapid thermal process, the temperature is rapidly increased up to T1 in a period for t1 seconds (about 10 seconds), wherein T1 is between 400.about.680.degree. C. Then the high temperature is maintained at T1 during the period of (t2-t1) (for about 10.about.40 seconds) and cooled down at about the 20.sup.th.about.50.sup.th second of the first rapid thermal process.
After performing the first rapid thermal process, a wet etching process is performed to remove the unreacted or remaining cobalt on the MOS transistor 10. Then a second rapid thermal process is performed to convert CoSi or Co.sub.2 Si into CoSi.sub.2 with low resistance so as to form the cobalt-containing salicide 20 in the interface between silicon and poly-silicon. Because of the low resistance of CoSi.sub.2 the resistance between cobalt and silicon, or cobalt and poly-silicon can be reduced.
In the prior art method of forming the salicide, the temperature of the first rapid thermal process allows cobalt and silicon to react and form a complex-phase structure which comprises not only CoSi or Co.sub.2 Si but also CoSi.sub.2 co-exiting with CoSi. As a result, CoSi or Co.sub.2 Si cannot be completely converted into CoSi.sub.2 with low resistance in the second rapid thermal process, increasing the resistance of the cobalt-containing salicide. Besides, using a transmission electron microscope to observe the cobalt-containing salicide formed by the prior art method, there was a defect found in it. Therefore, the yield of the salicide process may be reduced.